1. Field of the Invention
The present invention relates to elastic wave demultiplexers, and more particularly, to a Chip Size Package (CSP) elastic wave demultiplexer including a longitudinally-coupled-resonator-type elastic wave filter portion.
2. Description of the Related Art
For example, communication devices, such as mobile phones, supporting Code Division Multiple Access (CDMA) including Universal Mobile Telecommunications System (UMTS), include duplexers installed in their radio-frequency (RF) circuits in order to simultaneously perform transmission and reception of signals. Such a duplexer is a demultiplexer including a transmission filter, a reception filter, and a matching circuit. The duplexer is required to have a small insertion loss within the pass band and a large attenuation near the pass band in each of the transmission filter and the reception filter.
Surface acoustic wave duplexers in which their transmission filters and reception filters are defined by surface acoustic wave filters have been put into practical use. The reception filters in the duplexers have been required to have a balun function in recent years in order to omit separate baluns in RF circuits in mobile phones. Accordingly, surface acoustic wave duplexers in which the reception filters are defined by balanced longitudinally-coupled-resonator-type surface acoustic wave filters having a balanced-to-unbalanced conversion function are installed in the RF circuits in mobile phones (see, for example, Japanese Unexamined Patent Application Publication No. 2003-249842).
Reduction in size of demultiplexers, such as duplexers, is required in communication devices including mobile phones in order to reduce the sizes of their RF circuits. CSP elastic wave demultiplexers are known as the compact demultiplexers. In the CSP elastic wave demultiplexers, elastic wave filter chips are flip-flop mounted on wiring substrates and the elastic wave filter chips are sealed with sealing resin.
In the CSP elastic wave demultiplexers, inductors used in the transmission filters and the reception filters, inductors and delay lines defining matching circuits, and other circuit elements can be formed on the wiring substrates in order to improve the filter characteristics of the transmission filter and the reception filters. In other words, the inductors and the delay lines can be made of wirings provided on the surface of the wiring substrates or provided in the wiring substrates.
However, when the inductors and the delay lines are provided on or in the wiring substrates, for example, electrical coupling can occur in the wiring substrates and/or the transmission filters and the reception filters can be poorly grounded. If the transmission filters and the reception filters are poorly grounded, signals outside the pass bands that should flow into the ground may not flow into the ground. As a result, the attenuations outside the pass bands in the transmission filters and the reception filters are degraded, thereby degrading the isolation characteristics of the elastic wave demultiplexers.
In particular, the attenuations outside the pass bands are more likely to be degraded in the longitudinally-coupled-resonator-type surface acoustic wave filters when the grounding is poor, as compared to ladder-type surface acoustic wave filters. Accordingly, the isolation is likely to be degraded especially in surface acoustic wave filters that include a longitudinally-coupled-resonator-type surface acoustic wave filter as their reception filters.